﻿212 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  lY, 
  the 
  views 
  of 
  Professor 
  Xewberry 
  are 
  given, 
  and 
  issue 
  is 
  taken 
  

   with 
  him 
  as 
  regards 
  the 
  age 
  of 
  the 
  entire 
  group. 
  But 
  little 
  can 
  be 
  said 
  

   of 
  the 
  series, 
  as 
  our 
  work" 
  did 
  not 
  carry 
  us 
  far 
  enough 
  to 
  correlate 
  it 
  

   thoroughly 
  with 
  over- 
  as 
  well 
  as 
  underlying 
  strata. 
  It 
  can 
  be 
  said, 
  

   hoxsever, 
  that 
  it 
  is 
  entirely 
  conformable 
  with 
  the 
  Fox 
  Hills. 
  The 
  

   general 
  character 
  of 
  the 
  country 
  is 
  one 
  that 
  agrees 
  fully 
  with 
  the 
  litho- 
  

   fogical 
  constitution 
  of 
  the 
  beds, 
  and 
  very 
  similar 
  to 
  that 
  produced 
  by 
  

   the 
  same 
  formation 
  at 
  other 
  localities. 
  It 
  is 
  to 
  be 
  regretted 
  that 
  the 
  

   southern 
  extension 
  could 
  not 
  have 
  been 
  followed 
  sufficiently 
  far 
  to 
  prove 
  

   of 
  interest 
  paheoutologically. 
  From 
  the 
  evidence 
  obtained 
  by 
  Professor 
  

   Cope 
  farther 
  south, 
  and 
  the 
  negative 
  evidence 
  in 
  our 
  own 
  district, 
  there 
  

   is 
  scarcely 
  any 
  doubt 
  but 
  that 
  the 
  position 
  assigned 
  to 
  this 
  group 
  at 
  the 
  

   base 
  of 
  the 
  Tertiary 
  is 
  the 
  correct 
  one. 
  Eoom 
  is 
  left 
  for 
  doubt 
  only 
  by 
  

   the 
  absence 
  of 
  fossils. 
  Our 
  march 
  through 
  that 
  region 
  was 
  necessarily 
  

   a 
  hurried 
  one, 
  and 
  but 
  little 
  time 
  could 
  be 
  spent 
  at 
  any 
  particular 
  local- 
  

   ity. 
  Had 
  we 
  been 
  able 
  to 
  make 
  investigations 
  more 
  in 
  detail, 
  we 
  proba- 
  

   bly 
  should 
  have 
  found 
  our 
  conclusions 
  sustained 
  by 
  palteontological 
  evi- 
  

   dence. 
  

  

  Volcanic 
  rocJ:s.—Of 
  these 
  we 
  have 
  essentially 
  three 
  groups 
  in 
  our 
  dis- 
  

   trict: 
  (1) 
  the 
  trachorheites, 
  (2) 
  the 
  porphyritic 
  trachytes, 
  and, 
  (3) 
  the 
  

   basaltic 
  group. 
  The 
  first 
  covers 
  large 
  areas, 
  extending 
  in 
  Southern 
  

   Colorado 
  in 
  one 
  unbroken 
  mass 
  over 
  about 
  7,000 
  square 
  miles. 
  This 
  

   great 
  deposit 
  I 
  had 
  occasion 
  to 
  study 
  during 
  three 
  successive 
  years. 
  It 
  

   was 
  found 
  to 
  be, 
  lithologically 
  and 
  geognostically 
  speaking, 
  of 
  great 
  uni- 
  

   formity. 
  In 
  1874 
  I 
  found 
  a 
  region 
  which 
  bore 
  evidence 
  of 
  having 
  been 
  

   the 
  main 
  point 
  of 
  outflow, 
  for 
  at 
  least 
  a 
  very 
  large 
  portion 
  of 
  the 
  vol- 
  

   canic 
  material.* 
  Neither 
  in 
  1873 
  nor 
  1875 
  was 
  any 
  other 
  locality 
  found 
  

   that 
  could 
  at 
  all 
  be 
  considered 
  as 
  having 
  been 
  at 
  one 
  time 
  the 
  center 
  

   from 
  which 
  large 
  areas 
  were 
  overflowed 
  by 
  the 
  volcanic 
  material. 
  

   Throughout 
  the 
  region 
  of 
  the 
  Uncompahgre 
  Mountains 
  (with 
  the 
  excep- 
  

   tion 
  of 
  the 
  locality 
  above 
  mentioned) 
  the 
  trachorheites 
  show 
  a 
  regular 
  

   stratification, 
  on 
  a 
  grand 
  scale, 
  however. 
  At 
  many 
  points 
  the 
  single 
  

   strata 
  or 
  " 
  flows" 
  can 
  be 
  traced 
  for 
  miles, 
  or, 
  if 
  not 
  traced, 
  can 
  be 
  

   recognized 
  by 
  their 
  lithological 
  character. 
  The 
  present 
  position 
  of 
  well- 
  

   determined 
  volcanic 
  strata 
  is 
  a 
  strong 
  argument 
  for 
  the 
  theory 
  which 
  

   claims 
  a 
  gradual 
  rise 
  of 
  the 
  mountain 
  ranges 
  since 
  the 
  cessation 
  of 
  the 
  

   earlier 
  volcanic 
  activity. 
  We 
  find 
  on 
  high 
  ranges, 
  on 
  peaks 
  reaching 
  an 
  

   elevation 
  of 
  1-4,000 
  feet 
  above 
  sealevel, 
  the 
  strata 
  as 
  distinctly 
  marked 
  

   as 
  6,000 
  feet 
  lower 
  down. 
  We 
  can 
  scarcely 
  assume 
  that 
  the 
  region 
  of 
  

   outflow 
  was 
  at 
  one 
  time 
  so 
  high 
  that 
  it 
  occupied 
  a 
  sufhciently 
  elevated 
  po- 
  

   sition 
  to 
  send 
  its 
  flows 
  for 
  a 
  distance 
  of 
  more 
  than 
  a 
  hundred 
  miles, 
  where 
  

   they 
  now 
  are 
  found 
  at 
  an 
  altitude 
  over 
  12,000 
  feet. 
  It 
  is 
  more 
  reasonable 
  

   to 
  suppose 
  that, 
  at 
  the 
  period 
  of 
  the 
  massive 
  eruption, 
  the 
  ranges 
  now 
  

   composed 
  of 
  trachorheites 
  were 
  lower, 
  and 
  have, 
  since 
  that 
  time, 
  changed 
  

   their 
  absolute 
  elevation. 
  During 
  the 
  outflows 
  the 
  surface 
  of 
  the 
  country 
  

   was 
  already 
  corrugated, 
  as 
  is 
  shown 
  by 
  the 
  outcrops 
  of 
  underlying 
  met- 
  

   amorphic 
  rocks. 
  It 
  might 
  at 
  first 
  sight 
  be 
  supposed 
  that 
  the 
  volcanic 
  

   material 
  had 
  been 
  the 
  agent 
  X)roducing 
  a 
  metamorphosis, 
  which 
  now 
  we 
  

   would 
  find 
  at 
  places 
  best 
  adapted 
  for 
  exposures. 
  Ample 
  evidence 
  has 
  

   been 
  obtained, 
  however, 
  more 
  particularly 
  along 
  the 
  northern 
  border 
  of 
  

   the 
  Quartzite 
  Mountains, 
  that 
  whatever 
  local 
  ftifluence 
  the 
  hot 
  lavas 
  of 
  

   that 
  period 
  may 
  have 
  had, 
  they 
  have 
  not 
  been 
  productive 
  of 
  unj 
  exten- 
  

   sive 
  metamorphosis 
  of 
  older 
  formation. 
  Wherever 
  shales 
  and 
  sand- 
  

   stones 
  have 
  been 
  found 
  in 
  immediate 
  contact 
  with 
  the 
  volcanic 
  rocks, 
  

  

  * 
  Comp. 
  Report 
  Uuitecl 
  States 
  Geological 
  Survey, 
  1874, 
  p. 
  208. 
  

  

  